In a radio communication system, certain constraint conditions are imposed on the transmit power of radio signals, and each antenna of many radio remote unit (RRU) products is equipped with an independent power amplifier. Because the output power of a power amplifier is limited, the transmit power of each antenna should not exceed a specified maximum transmit power of the antenna. Many RRU products implement power sharing between antennas. Therefore, the transmit power of each RRU should not exceed a specified maximum transmit power of the RRU. When a large number of transmit antennas are used in a cell, considering costs and other aspects, multiple RRUs may be used together, which requires that the transmit power of each cell should not exceed a specified maximum transmit power of the cell. Because a uniform power supply is provided for multiple cells of a base station, another possible constraint condition is that the transmit power of each base station should not exceed a specified maximum transmit power of the base station.
The transmit power constraint conditions may be collectively represented by a power-limited antenna set. Regarding an antenna power limit, an RRU power limit, a cell power limit, and a base station power limit, each power-limited antenna set respectively corresponds to a transmit antenna, all transmit antennas of an RRU, all transmit antennas of a cell, and all transmit antennas of a base station. Different power-limited antenna sets may have different maximum transmit power limits. The total transmit power of all antennas in each power-limited antenna set should not exceed a corresponding maximum transmit power.
In a practical system, the base stations of different specifications may be used to implement different power limits. For example, in a heterogeneous network, the maximum transmit power of a macro cell antenna is different from the maximum transmit power of a micro cell antenna. Therefore, the transmit power constraint conditions in a radio communication system may be completely represented by a combination of various power-limited antenna sets.
To further enhance capacity of a radio communication system, a coordinated multi-point (CoMP) technology in researches of standards such as Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) attracts more and more attention and is extensively studied. When the CoMP technology is applied between base stations, the path loss from each user to each base station varies sharply, and the power allocated to each power-limited antenna set after precoding varies sharply. Therefore, the transmit power of each power-limited antenna set, which is calculated according to a general method, tends to exceed the maximum transmit power limit easily.
Generally a relatively practical technical solution in engineering to solve such a problem is reducing the powers of all base station antennas evenly to satisfy the transmit power constraint conditions in the radio communication system.
After a basic precoding matrix W is obtained (the precoding matrix includes precoding matrices of M subbands, that is, Wm (m=1, 2, . . . , M)), the precoding matrix is multiplied by a certain power reduction factor α less than 1, that is, the transmit powers of all base station antennas are reduced to a certain extent evenly:Wm=α·Wm.
The easiest way of setting the power reduction factor is to reduce the power by 3 decibels (dB) directly:
      W    m    =            1              2              ·                  W        m            .      
A better way is to first estimate a transmit power Pn of each antenna according to a precoding matrix, and then set the power reduction factor by reducing the largest value to satisfy the requirement of the maximum transmit power limit Pmax:
      W    m    =                              P          max                            max        ⁢                  {                      P            n                    }                      ·                  W        m            .      
Although the maximum transmit power limit can be met by simply reducing the transmit powers of all base station antennas to a certain extent evenly, the technical solution reduces the transmit powers of all base station antennas evenly without discrimination. Therefore, the transmit power of the power-limited antenna set whose transmit power does not exceed the maximum transmit power limit is reduced to a lower level, which leads to unnecessary loss of performance of coverage and capacity and deteriorates network performance such as coverage and capacity apparently.